An inkjet ink containing an aqueous pigment as a colorant has various advantages when it is used for printing on plain paper, such as fewer occurrences of bleeding, high image density, and less occurrences of strike through. In the case a deposition amount of the ink is large, such as printing photographs or diagrams, on plain paper, however, the printed plain paper tends to cause back curling. The back curling is a phenomenon that paper is warped to the opposite side to the side of the printed surface. If the back curling of plain paper occurs just after printing, a transfer failure of the paper occurs in the inkjet printer (within the device) during the paper conveying process. The conveyance of the paper is very difficult especially when the back curling of the paper occurs just after high speed printing or double-sided printing.
Therefore, it has been desired to develop an inkjet ink that gives no or less back curling when it is used with a large deposition amount thereof on plain paper, such as in printing photographs and drawings on plain paper. Especially a high-speed inkjet printer equipped with a line-head has a higher need for such the ink compared to a serial printer.
As the known technique for suppressing curling of paper, there is a method for applying an alcohol solution to paper prior to recording with an ink, allowing the paper to be substantially dry in a recording position, and carrying out recording with the ink (see PTL1). PTL1 teaches that hydroxyl groups of the alcohol solution are bounded to hydroxyl groups present at bonding points of hydrogen bonds between cellulose fibers of paper to block water molecules in the ink with hydrophobic groups of the alcohol solution. However, this method does not give the effect when a large amount of an aqueous ink is ejected during high speed printing, and therefore it has not been able to prevent curing of paper immediately after printing.
Moreover, there is a recording method for ejecting an ink and a reaction solution that reacts with the ink, and in this method the reaction solution is ejected to land on an opposite surface of a recording medium to the surface where the ink is recorded, corresponding to the data identical to the data for recording the ink (see PTL2). In accordance with this method, however, a configuration of a recording device for use is complex, and curling of printed paper cannot be prevented unless a reaction liquid formed of the substantially same formulation to that of the ink is ejected in the same or similar amount to that of the ink. Therefore, this method is economically disadvantageous. As a large amount of water is contained on both sides of paper when a solid image is printed on almost the entire area of the paper, the paper loses its stiffness, which makes it to difficult to convey the paper.
Further, PTL3 discloses an inkjet ink composition containing diglycerin or polyglycerin in combination with polyethylene glycol monoalkyl ether, and PTL4 discloses an inkjet ink composition containing polyethylene glycol monomethyl ether. This inkjet ink composition however does not exhibit any effect of preventing curling of paper when the aqueous ink is ejected in a large amount with high-speed printing. Therefore, the proposed inkjet ink composition does not satisfy the demands for preventing curling of paper just after being printed. Since the organic solvent contained in the inkjet ink composition has low equilibrium moisture content, moreover, the ink composition cannot secure ejection stability.